Each day hundreds of scheduled flights operated by the major airlines, such as United Airlines, America Airlines, Delta, Northwest, Luftansa, Aer Lingus, and VietNam air traverse routes between cities throughout the world. In addition, cargo carriers, such as the United Post Office, DHL, Federal Express, and United Parcel Service fly routes throughout the world on a daily basis. Aircraft on these regularly scheduled flights travel generally predictable routes at generally predictable times. In addition to these commercial flights, there are numerous charter and general aviation flights, amounting to thousands of aircraft aloft each day, covering a large geographic area and encountering a wide variety of atmospheric and weather conditions at different locations and altitudes at different times of the day.
Weather conditions affect many aspects of human life such as agriculture production and famine, public safety, transportation, tourism and communications. Thus, improved weather forecasting has many potential benefits. The combination of ground-based monitoring and satellite imagery have substantially enhanced weather prediction, however, weather forecasting can be further enhanced with more accurate weather data of conditions aloft.
In addition, aircraft generally use static barometric pressure meters for determining altitude above sea level or relative to ground level. Such pressure altimeters operate by measuring local static pressure and comparing the measured pressure to a lookup table or calibration curve (correlating barometric pressure to altitude) in order to determine the corresponding altitude. This measure of altitude is referred to as pressure altitude because it is based upon a reading from an atmospheric pressure measurement device such as a static port and pressure transducer. A pressure altitude measurement, however, may not reflect the true altitude of the aircraft because the measurement is based on the assumption that atmospheric pressure is solely a function of altitude. This assumption may be incorrect—as other factors may alter the atmospheric pressure. Thus, a reading of pressure altitude may vary from “true altitude”. Barometric pressure readings (and thus pressure altitude measurements) are affected by other atmospheric conditions such as wind speed and temperature. Thus, circularity problems arise when attempting to obtain a measure of atmospheric conditions as a function of altitude.